In doing some research, I found a thread where a dude put a Warmoth 28.5" neck on a Jazzmaster and said it intonated correctly. He said a 30" scale would yield 3/4" bridge drop:
http://www.offsetguitars.com/forums/vie ... hp?t=26110
The black one is a Jag with a Warmoth bari neck strapped to it. The Jag is a shorter scale than a Jazzy - jazzys are the same as a strat - the Warmoth neck is scaled to bolt straight to a strat, so that means you can bolt it straight to a Jazzy, no mods. I had to move the Jag bridge south, though - 3/4" to get the scale correct. That wasn't hard either. If you wanted the USACG scale neck on a jazzy body, you will have to move the Jazzy bridge south by maybe 3/4" to make it work, which will probably mess with your pickguard...
So - the Warmoth neck is probably the least hassle...I have my red Bass VI build setup for baritone just like the black Jag bari Conversion, and the 1.5" scale difference between the two means very little to the tone and playability - the Bass VI sounds perhaps slightly chunkier than the jag Conversion...the only thing with the Warmoth neck is that a) the fretboard is an overhanger(not very vintage-looking) and the headstock is a small strat-style pre-CBS shape, which looks a little odd with the Jag/Jazzy bodyshape...but not too bad! Both these instruments I tune baritone BEADF#B (often with dropped A) with D'Addario XL157's and they both sound huge with SD Antiquities(II's in the Bass VI, I's in the Jag)...
Maths:
A 30" scale neck means you will have 15" from nut to 12th fret. Therefore you will need 15" from 12th fret to bridge to get intonation correct.
A 28.5" scale neck means you have 14.25" from nut to 12th fret - therefore you need 14.25" from 12th fret to bridge to get intonation correct
So I hope this helps!"